When the White House was preparing to unveil its new wageprice guidelines program last October, it added -- almost as an afterthought -- a new "wage insurance" plan.

The idea was for a tax rebate that the government would offer workers as a guarantee that they wouldn't be caught short by following the new 7 percent guideline.

If inflation exceeded the 7 percent goal Carter set, workers who agreed to adhere to the guidelines would get a tax rebate to make up the difference. In effect, it would amount to "insurance" in case the guidelines didn't work.

The plan had two major attractions: First, it would serve as a last-minute "sweetener" for organized labor -- something to help coax union leaders into going along with the guidelines program.

And, second, because most of the guidelines' details already had been made public, the proposal would provide a fresh news peg for the president's announcement speech -- a little p.r. trick to help make a bigger splash.

Now, as Carter prepares to send the rebate plan to Congress, it's becoming clear there wasn't enough thought -- even afer-thought -- to the wage-insurance proposal, and the plan could run into a snag on Capitol Hill.

To begin with, the Treasury experts who were told to draft the package have had to put together a complex -- and somewhat spotty -- proposal. (The tax writers were not consulted before the rebate plan was nnounced.)

There are numerous technical problems involving who should be eligible for the rebate, how to deal with special groups such as selfemployed persons and farmers, and how to prevent fraud and abuse.

More important, the proposal appears to be headed for trouble in the House Ways and Means Committee, where both Democrats and Republicans seem to be unenthusiastic -- and even suspicious -- about it.

Panel leaders fear that unless the committee can come up with a saleable compromise, the exercise could turn into the sort of melee that characterized Congress' handling of the 1978 tax bill -- perhaps with similar results.

Some conservatives already are talking about turning the bill into a vehicle for enacting "indexing" -- automatic inflation adjustments in the income tax system, which Carter opposes as itself inflationary.

And Democrats on the panel are fearful there will be so much pressure in committee to expand the coverage of the rebate that the proposal will mushroom into a sort of general income-tax cut, which no one up there wants.

As it stnds now, the White House still is planning to go ahead with the plan. Carter is scheduled to approve the final version of the rebate proposal today, with the Treasury likely to make it public sometime next week.

The administration already has had to put a "cap on the plan -- to hold the cost of it down -- by limiting the rebate to no more than 3 percent of the first $20,000 of a worker's income, or $600 a taxpayer.

That means workers now would be covered only through an inflation rate of up to 10 percent -- and even then, upper-middle income persons wouldn't be held fully harmless.

That may make sensible economics. Few forecasters really expect inflation to exceed, say, 3 percent in 1979. But it may not make great politics. It was pressure from middle-income taxpayers that burst Carter's big tax bill in 1978.

But committee sources expect still more changes to come.

Reps. James R. Jones (D-Okla.) and Joseph L. Fisher (D-Va.) are working on one proposal that would offer workers a flat $100 rebate for each percentage point that inflation exceeded the 7 percent target.

And the staff of the Joint Committee on Taxation is considering other possibilities. But the technical problems remain in all. At this point, there's no telling where the legislation will come out.

The administration already has begun lobbying intensively for the proposal. Domestic adviser Stuart E. Eizenstat and White House economist Charles L. Schultze have made frequent trips to Capitol Hill to explain the plan.

But most analysts -- including Carter's own lobbyists -- still expect the plan to open up more problems for the White House than it solves. "if the Hill runs away with this," says one official, "we'll really be in trouble."

It wouldn't be so bad if the administration had studied the notion thoroughly before coming out with it publicly, but insiders concede this one was taked on hastily, almost entirely as a political sugarplum.

The difficulty is, if the Ways and Means Committee can't find a compromise for adopting the new rebate plan, Carter may find he'll have to ask Congress to drop it altogether, rather than risk any of the likely alternatives.

If so, that would give labor another excuse to sidertrack the wage-price guidelines. And what started out as a sweetener could sour the whole plan.